The present invention relates to appending data onto magnetic tape.
Data is written onto magnetic tape for storage and subsequent retrieval. Generally, data is written onto the tape in contiguous blocks separated by gaps. The gaps delimit blocks and assist in finding the beginning of a particular block during read operations. When a steady stream of data is available, blocks are written onto the tape continuously. However, new blocks of data are often appended after existing blocks as data becomes available. These append operations may write on clean tape or may write over old existing data that is no longer needed.
A typical tape access system includes a tape head with a write module and a read module trailing the write module in the direction of tape travel. The write module contains a plurality of write elements for simultaneously writing many data tracks. Similarly, the trailing read module will include a plurality of read elements for simultaneously reading the data tracks. Following each write element with a read element permits immediate read after write to verify that data has been correctly written onto the tape. A servo controller controls one or more motors for moving the tape past the tape head in a particular direction and at a particular speed. Write circuitry converts data into write module write signals. Read circuitry converts read module read signals into data. The read circuitry also detects the end of a block and signals a control unit, which generates a gap-out signal to the servo controller. The servo controller remembers the tape location corresponding to the gap-out signal. The servo controller then generates a gap-in signal whenever the tape is a preset distance before the remembered location of the end of the block.
During an append operation, the gap spacing may be set by any of several methods. A first method is to wait a preset period of time after receiving the gap-in signal from the servo controller. The accuracy of this method depends on precise timing of the gap-in signal. A second method is to begin writing appended data after the trailing read module reads the end of the last data block. While this method is more accurate than relying on the gap-in signal, the gap distance can be no less than the distance between read element and write elements in the direction of tape travel.
The desire to increase the density of information stored on magnetic tape drives the need to narrow the gap distance. Narrowing the gap distance by decreasing the distance between a write element and the corresponding read element increases the manufacturing cost of the read head and increases the possibility of significant cross-talk or direct coupling of the magnetic signal produced by the write element into nearby read elements. Narrowing the gap distance in systems relying on the gap-in signal increases the cost and complexity of the servo controller due to the tighter timing tolerance required to prevent either overwriting the end of an existing block of data or not overwriting old, unwanted data under the appended data block.
What is needed is to achieve higher tape data density by shortening the gap distance between data blocks without increasing the possibility of lost or erroneous data and without substantially increasing the cost or complexity of the tape access system.
It is an object of the present invention to reduce the gap distance between existing data and appended data.
It is another object of the present invention to increase the accuracy of an appended data block starting location.
It is still another object of the present invention to reduce the possibility of overwriting the end of an existing block when appending new data.
It is yet another object of the present invention to reduce the possibility of not overwriting old, unerased data when appending new data.
It is a further object of the present invention to reduce tolerance requirements for the gap location signal generated by the servo controller.
In carrying out the above objects and other objects and features of the present invention, a method for appending data to existing data on magnetic tape is provided. The tape travels past a tape head having a write module between a leading read module and a trailing read module in the direction of tape travel. Data is read with the leading read module. The end of the existing data is determined from the data read by the leading read module. A gap period is delayed to introduce a gap between the existing data and appended data. The appended data is written following the gap with the write module. The written data is verified by reading with the trailing module.
In an embodiment of the present invention, a servo controller generates a signal indicating the approach of the end of existing data. The method further includes waiting to read existing data until the signal is generated.
In another embodiment of the present invention, a data selector selects between reading from the leading read module and from the trailing read module. The method further includes selecting the leading read module before reading existing data and selecting the trailing read module after determining the end of the existing data.
In still another embodiment of the present invention, the tape direction may be reversed. The leading read module may then function as the trailing read module and the trailing read module may then function as the leading read module when data is to be appended in the reverse tape direction.
A system for appending data to existing data on magnetic tape is also provided. The system includes a tape head for accessing the tape. The tape head has a write module between a leading read module in front of the write module in the direction of tape travel and a trailing read module in back of the write module. A tape drive moves magnetic tape past the tape head. A data write circuit generates write module write signals from received data. A data read circuit generates transmitted data from read module read signals. A control unit enables the data read circuit to read with the leading read module, enables the data read circuit to determine the end of the existing data, delays a gap period to introduce a gap between the existing data and appended data, enables the data write circuit to write appended data, and enables the data read circuit to read the appended data with the trailing read module.
In an embodiment of the present invention, the system includes a servo controller to control the tape speed and direction and to generate a gap-in signal indicating the approach of the end of existing data. The control unit waits to read existing data until the gap-in signal is generated. The control unit generates a gap-out signal based on detection of the end of existing data by the data read circuit.
In another embodiment of the present invention, the system includes a data selector forwarding to the read circuit either read signals from the leading read module or from the trailing read module based on a select control signal. The control unit sets the select control signal to forward leading read module read signals before enabling the data read circuit to read existing data and to set the select control signal to forward trailing read module read signals after the data read circuit determines the end of the existing data.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.